1. Field of the Invention
This invention relates generally to aqueous polymer latex compositions, and more specifically to polymer latex compositions including residual formaldedhyde, such as self-crosslinking polymer latex compositions including polymers having N-methylol functionality for crosslinking,
2. Brief Description of the Prior Art
Formaldehyde is an undesired by-product present in some polymer latex compositions For example, formaldehyde may be generated from sodium formaldehyde sulfoxylate, a reducing agent used in some redox polymerization systems In addition, formaldehyde is sometimes generated in curing of self-crosslinking polymer latex compositions.
Self-crosslinking aqueous polymer latex compositions are known for a number of different applications. Self-crosslinking polymers can be crosslinked without an additional chemical crosslinking agent. Frequently, an end-use composition is prepared including the self-crosslinking aqueous polymer latex as a binder, and the composition is subsequently crosslinked into a three-dimensional coating or article, as by application of heat. The use of a self-crosslinking polymer latex simplifies the preparation of the end-use composition by reducing the number of components, eliminating the need to mix a separate catalyst or crosslinking agent immediately before use, although a catalyst may be premixed in the composition In addition, the use of a self-crosslinking polymer may permit the preparation of a preformed, incompletely cured article or stock material, which can be subjected to additional processing steps before cure, adding flexibility to the manufacturing process Use of a self-crosslinking polymer latex may reduce the likelihood that an undesirable monomeric catalyst or unreacted crosslinking agent will subsequently leach out of or migrate from the end-use product into its environment.
One type of self-crosslinking aqueous polymer latex includes a N-methylol functional residue. This type of polymer latex is polymerized from a monomer mixture which includes a copolymerizable, ethyenically unsaturated N-methylol-functional monomer, such as N-methylolacrylamide. Polymer latexes prepared using N-methylolacrylamide have been used, for example, as binders for non-woven products, as bases for oil-well drilling fluid compositions, as viscosity modifiers for injection water for enhanced oil recovery, in preparing ion-exchange resins, as chemically modified cellulose for paper, in making shoeboard for shoes, in making synthetic leather, in making photographic paper, in preparing mortar mix, and in preparing baking enamels.
N-methylolacrylamide is typically prepared by reaction in aqueous media of ac with formaldehyde: ##STR3##
Preparations of N-methylolacrylamide can include excess unreacted acrylamide to reduce the equilibrium formaldehyde level in the preparation. The rate constant for the preparation reaction is pH-dependant, as is the dissociation reaction for the polymerized residue of the N-methylol acryamide. See, e.g., H. A. Pederson, Handbook of Fiber Science and Technology, Volume II, Part A (Marcel Dekker, Inc. 1983) pp. 47-327.
Formaldehyde itself is undesirable as a contaminant in many products, and efforts have been made to reduce ambient formaldehyde levels in products such as urea-formaldehyde insulation and in other building products such as chip-board and the like in which urea-formaldehyde-type adhesives are used. Formaldehyde can be generated by dissociation of polymerized N-methylol acrylamide, such as that found in some aqueous polymer latex compositions used to prepare texile finishes and the like, as well as during heating of the end-use composition to prepare a product. Because the evolved free formaldehyde is believed hazardous, it is desirable to minimize its generation in such compositions. Commercial self-crosslinking aqueous polymer latex compositions may include several hundred parts per million or more of formaldehyde from various sources.
A self-crosslinking aqueous polymer latex including the residue of N-methylolacrylamide is a potential source of formaldehyde, and it is strongly desirable to minimize the amount of free formaldehyde generated from the N-methylolacrylamide residue during storage and use of the corresponding self-crosslinking aqueous polymer latex and end-use products which are made using that latex Efforts have been made to reduce formaldehyde emissions from methylolated resins generally. For example, U.S. Pat. No. 4,735,851 discloses treating formaldehyde-containing resin with peroxide to reduce formaldehyde liberation. U.S. Pat. No. 4,670,184 discloses a 4,6-diamino-1,3,5-triazin-2-yl compound as a formaldehyde binding agent in molding resins U.S. Pat. No. 4,525,492 discloses adding urea, melamine or dicyandiamide as a coreactant in phenolic foams to reduce free formaldehyde. U.S. Pat. No. 4,472,165 discloses the use of formaldehyde scavengers, such as urea and ethyleneurea, in the formulation of foams for reducing free formaldehyde in fabrics treated with methylolated resins or dimethylol compounds for imparting wrinkle resistance. U.S. Pat. No. 4,323,624 discloses treating fibers with an aldehyde-based resin, such as a urea-aldehyde resin, and a nitroalkanol to reduce free formaldehyde. European Patent Application 0 302 289 A discloses a process for reducing formaldehyde emissions from acid-hardening lacquers by adding a compound containing an acidic alpha-methylene group and an amide nitrogen, such as cyanoacetamide. Japanese Patent Publication 49/106 588 discloses addition of urea, thiourea or ethyleneurea to urearesins to lower free formaldehyde content French Patent Application 2 575 754 A discloses addition of an alpha-methylated ketone to a formaldehyde-type condensation resin to lower free formaldehyde.
In general, it is desirable that methods of mimimizing the formaldehyde generated avoid undesirable side effects, such as the formation of colored products which detract from the appearance of the end product It is also desirable that no more than low levels of formaldehyde be generated in self-crosslinking methylol-functional polymer latex compositions during storage prior to use. It is also important that otherwise desirable application properties of compositions including self-crosslinking polymer latexes be maintained when such compositions are reformulated to reduce formaldehyde evolution.
Efforts have been made to reduce liberation of formaldehyde upon drying or curing of synthetic polymer films containing residues of N-methylolacrylamide. For example, U.S. Pat. No. 4,473,678 discloses self-crosslinking aqueous dispersions of synthetic resin which contain N-methylolamide groups, but which additionally contain both from 2 to 10 percent by weight of a hydroxyalkyl ester, such as 2-hydroxyalkyl acrylate, in the resin, and urea in the aqueous phase. U.S. Pat. No. 4,524,093 discloses a fabric coating composition with low formaldehyde evolution during curing which includes an aqueous emulsion of alkyl acrylate monomers copolymerized with acrylonitrile, itaconic acid, and N-methylolacrylamide, and containing an imidazolidone curing resin and a metal chloride-type acid catalyst. Similarly, U.S Pat. No. 3,590,100 discloses driving to completion reactions of the type which produce a dimethylol alkyl carbamate for textile finishing by reacting the free formaldehyde coproduct by reaction with added ethylene urea, propylene urea or glyoyxyl and urea. German Offenlegungsschrift 25 38 015 discloses removal of free formaldehyde from microcapsule dispersions by addition of hydroxy, methoxy, ethoxy, methyl or ethyl derivatives of 5- or 6-membered rings derived from urea or of 2-oxo-isoxazolidine.
With respect to aqueous dispersions of polymerized ethylenically unsaturated monomers, Australian Patent Application 31608/84 published 12/24/87 discloses the use of ethylene urea, mono- and dihydroxyethyleneurea and sundry six-member ring compounds including the --NHC(O)NH-- radical as a formaldehyde scavenger in aqueous plastics dispersions based on a crosslinkable polymer of ethylenically unsaturated monomers containing N-methylolamide and/or N-methylol ether-amide groups.
While progress has been made in reducing formaldehyde evolution during drying and cure of self-crosslinking, methylol-functional polymer latex compositions, there is still a significant need for a self-crosslinking aqueous polymer latex compostion in which only low levels of formaldehyde are generated during extended storage, and evolves low levels of formaldehyde during drying and cure, while retaining important application properties.